The present invention relates to a process and to an apparatus for the fast measurement of energy. It in particular makes it possible to repetitively measure the energy supplied by a pulsed laser in a very broad spectral range from ultra-high frequencies to ultraviolet.
The presently used apparatuses or sensors for measuring energy supplied in particular by a laser beam are directly based on the sensors or detectors used in infrared spectrometry and whose characteristics have been optimized. These optimized sensors, not initially designed for this type of application, do not necessarily form the best possible choice for this type of application.
They are essentially constituted by pyroelectric sensors or optionally bolometers having three separate parts. As can be gathered from FIG. 1, these sensors comprise a thin layer of an absorbent material 2, whose absorption spectrum must be as wide as possible, i.e. it must absorb energy from a laser pulse corresponding to a wavelength ranging from the more or less remote infrared to the ultraviolet. This thin layer of absorbent material 2 converts the energy of the laser pulse into heat. Although the conversion time of the energy of the laser photons into thermal energy is very short, generally approximately 1 picosecond, the layer of absorbent material 2 must, by its very nature, be physically separated from the second part 4 constituting the sensors. This physical separation leads to a first limitation of the response time of the sensor linked with the transfer time T.sub.1 of the thermal energy supplied by the thin layer of absorbent material 2 to the second part 4 of the sensor.
This second part 4 makes it possible to convert the thermal energy into an electrical signal proportional thereto. This second part is generally, but not exclusively, formed from a pyroelectric ceramic material, at whose terminals appears a potential difference, in open circuit, which is proportional to its heating. This potential difference is then transmitted to an electrical measuring apparatus.
In view of the fact that the pyroelectric ceramic has a very high internal impedance, its output impedance must be lowered via an impedance reducing circuit generally formed by a MOS transistor 6. To avoid the interception of unwanted electrical signals, the MOS transistor must be positioned as close as possible to the pyroelectric ceramic and must be integrated into the sensor. This MOS transistor forms the third part of the sensor. The resistor R connected to the terminals of the pyroelectric ceramic 4 represents the leakage resistance of the grid 8 of the transistor and capacitor C.sub.d connected to the drain and to the grid of the transistor by means of resistor R represents the bypass capacitor of the transistor supply source. The measuring signal is collected at S.
Moreover, due to the fact that the pyroelectric ceramic has a very high impedance, the latter unfortunately has a high parallel capacitance. This high capacitance, represented in dotted lines in FIG. 1 and carrying reference C, introduces a time constant T.sub.2, which is much higher than the transfer time T.sub.1 of the thermal energy of the thin layer of absorbent material 2 to the pyroelectric ceramic 4. Thus, this time constant varies from 10 to 30 ms, whilst the transfer time T.sub.1 varies from 1 .mu.s to 1 ms. Consequently, the high sensitivity of such a sensor can only be obtained close to a high time constant, which limits the maximum repetition frequency of the energy measurements supplied in particular by a laser beam at 100 Hz. This is inadequate for repetitively measuring the energy supplied by a modern pulsed laser, which reaches a frequency of several kHz.
It should be noted that the time interval separating two successive measurements has nothing to do with the minimum duration of a laser pulse which can be detected by the sensor. The latter is equal to the conversion time of the energy of the photons into heat, i.e. approximately 1 picosecond.
Moreover, such a sensor or detector has a limited spectral response in the remote infrared. Moreover, the complex structure of this sensor leads to a high cost. Finally, the energy absorption takes place in a very small volume constituted by the layer of absorbent material. Thus, the energy density supplied by the laser beam is high there, which leads to a rapid deterioration of the layer of absorbent material so that it is periodically necessary to regenerate the latter by applying a coating to the surface of the absorbent material layer. This regeneration of the latter then requires a complete recalibration of the sensor. Moreover, when the laser energy density is very high, the sensor may be completely destroyed, so that it is necessary to replace the latter, involving high costs.